Technology update

Oct 14, 2003

Nanoindenter cuts up nanowires

Researchers from the University of South Carolina, US, have combined an atomic force microscope (AFM) with a nanoindenter to measure the mechanical properties of silver nanowires. The technique could also have applications in manufacturing nanodevices as it can permanently change the shape of the nanowires.

“To our knowledge, we are the first to make nanoindentations on a single nanowire,” researcher Xiaodong Li told nanotechweb.org. “Our findings are of great importance for both mechanical property measurement and integration/manufacturing of nanodevices. Our experience may help industry to develop nanoscale machining tools.”

Li and colleagues used a pyramidal nanoindenter tip both to image a silver nanowire and to indent it. Monitoring the load and displacement of the indenter during indentation gave information about the mechanical properties of the nanowire. A silver nanowire around 42 nm in diameter and 4 µm long, for example, had a hardness of 0.87 +/- 4 GPa and an elastic modulus of 88 +/-5 GPa. These quantities agreed well with nanoindentation values for a bulk single crystal of silver.

According to Li, it’s better to use a nanoindenter than an AFM for indentation tests because the AFM tip cannot be perpendicular to the sample surface. This causes slip and friction between the tip and the sample, making it impossible for the AFM to measure
indentation load and displacement accurately. In addition, AFM cantilevers are not stiff enough to apply high indentation loads. “This is why we used a new approach - nanoindentation,” said Li. “We combined a nanoindenter with an AFM to directly visualize a silver nanowire and indent the wire in situ, allowing us to directly, accurately measure the mechanical properties of the wire.”

As well as measuring the wire’s mechanical properties, the scientists cut the nanowire in two by applying a high indentation load. They also used the indenter to make an array of nanoscale slots of different sizes and shapes. The team varied the depth and size of the indents by applying different indentation loads, and changed the shape by using indenters with different geometries, such as the corner of a cube and conical and spherical tips. The researchers believe such indents could have applications as cells for molecular electronics and drug delivery, slots for integration of the nanowires into nanodevices, and defects for tailoring the properties of the nanowires.

Li says it’s difficult to achieve nanomachining on nanowires by conventional lithography and synthesis methods. “The combination of a nanoindenter and an AFM makes the nanoscale mechanical property measurement, machining and integration of nanobuilding blocks possible,” he added.

Now, the researchers, who reported their work in Nano Letters, hope to develop the technique further and achieve nanoscale forging and shaping.